Process of electrophoretic deposition using symmetrical alternating current



July 9, 1968 R. BARRETT ETAL 3,392,101

PROCESS OF ELECTROFHORETIC DEPOSITION USING SYMMETRICAL ALTERNATING CURRENT Filed July 17, 1964 2 SheetsSheet 1 FIG. 1. FIG. 2.

9 I 0 I I HHHHH.

J07!!! 1076 pg p1 5 ll/fffd f FIG. 3.

Romuo L. BAmze'r'r Kiarn M- 5 I ATTO N YS July 9. 1968 R. BARRETT ETAL 3,39

PROCESS OF ELECTROPHORETIC DEPOSITION USING SYMMETRICAL ALTERNATING CURRENT Filed July 17. 1964 2 Sheets-Sheet 1':

' FIG. 4.

FIG. 5.

mvfiuroks:

Rouuo L. BAERE rr Kurn M- Hoes ATroR-nEYs United States Patent ABSTRACT OF THE DISCLOSURE Deposition of a film of pigmented resin on a metal object by dispersing a resin having free acid groups and a pigment in an aqueous medium containing a base to form an aqueous dispersion of pigmented resin, forming an electrolytic cell using the metal object as an electrode of the cell and then subjecting the aqueous dispersion to electrophoresis using symmetrical alternating current voltage to deposit the pigmented resin on the metal object as an adherent film.

The present invention relates to the deposition of a film of pigmented resin on a metal object. In particular the present invention relates to the electrophoretic deposition of a film of pigmented resin on a metal object from an aqueous dispersion of said resin.

In co-pending application Ser. No. 261,564 of 1963 by I. K. Gentles et al., there is described and claimed a process for the deposition of a film of a pigmented resin on a metal object, which comprises incorporating a pigment into a resin having free acid groups, dispersing the pigmented resin in an aqueous medium containing a base to form an aqueous dispersion of said pigmented resin, forming an electrolytic cell using the metal object as the anode of the cell and subjecting the aqueous dispersion to electrophoresis whereby to deposit the pigmented resin on the metal object as an adherent film. The pigmented resin coated object is then normally stored.

As will be seen from the examples in this co-pending application, it has heretofore been considered necessary by the applicants for that co-pending application to use a direct current in order to achieve the deposition of the film. In contrast thereto it has now been found that the use of a symmetrical alternating current and voltage achieves particularly good results and has the advantages that it eliminates the necessity for rectification of the current taken from a mains supply and also as will be seen later, AC (alternating current) voltages of 200250 volts have been found most suitable, whereby there is no necessity for the use of step up and step down transformers with the electrophoretic cell. A further advantage is that Whereas using direct currents and voltages it is only possible to apply pigmented resin to one article which forms the anode of the cell with AC voltages and currents, pigmented resin can be deposited on both electrodes of the cell and therefore it is possible to coat two articles simultaneously with pigmented resin.

The present invention therefore comprises a process for the deposition of a film of pigmented resin on a metal object according to the process described and claimed in co-pending application Ser. No. 261,564 of 1963 by l. K. Gentles et al., in which the voltage applied to the electrophoretic cell is an AC voltage.

AC voltages which are desirable for use in electro phoretic cells according to the process of the present invention are generally greater than 150 volts, preferably greater than 200 volts and more preferably of the order "ice of 250 volts. Whereas the maximum voltage is not critical, voltages are substantially higher than 500 volts tend to cause electrolysis to take place in the cell with resultant pitting of the coating of pigmented resin on the metal object.

These voltages apply to electrodes from six inches to one foot apart. For greater separations of the electrodes, higher voltages are necessary to produce the same effect.

The current densities found desirable in the cell are of the order of 8-15 amps per sq. ft., more preferably 10- 15 amps per sq. ft. and optimally 1012 amps per sq. ft. of the electrodes. As with high voltages, high current densities which normally result from the application of high voltage also cause electrolysis in the cell and therefore are not desired.

The period during which current is passed through the electrophoretic cell is not particularly critical but from practical considerations, for simple objects i.e. objects having a non-complex configuration, a period of time from 30 secs. to 2 mins. is found generally applicable, for a period of 30 secs. to 1 mins. is preferable. For more complex articles, slightly longer times are generally advisable. It Will be realized that as the article becomes coated with pigmented resin the current density will decrease as the coated article becomes less conductive and therefore increased periods of application of the AC voltage to the cell will produce Very little improvement in the thickness of the film and will in fact generally detract from the quality of the film because of electrolysis which may take place.

It has been found when an AC voltage is applied to the electrophoretic cell of the present invention the articles coated with the pigmented resin have a relatively rough surface and a smooth surface, the rough surfaces correspond to the faces of the article which lay opposite each other in the electrophoretic cell and the faces having a smooth surface to the faces which lay away from each other in the electrophoretic cell. While this has no undue disadvantage when applying a primer coating to the articles, in that the primer-coating is eventually going to be covered by a surface coating, when it is intended that the coating as applied by the process of the present invention is to be a surface coating, then it is desirable to have a completely smooth surface on the article. To facilitate this, two methods of procedure may be followed.

In the first instance one may rotate the electrodes of the electrophoretic cell so as to avoid a surface of each of the electrodes permanently facing one another during the coating process or on the other hand one can make one of the electrodes a wire pole, i.e. form one electrode from a piece of Wire by which means it has been found that a continuous smooth coating is formed over all the surface of the other electrode. Thus, it is envisaged for continuous coating of metal objects to form an electrophoretic cell comprising a bath of the pigmented resin dispersion having as one pole thereof an endless wire which continuously runs through the bath and is passed through a cleaning bath to remove adherent pigmented resin and returned to the cell and to pass continuously through the bath as the other electrode objects to be coated.

It is highly desirable in order to obtain a good coating of pigmented resins upon the articles to continuously agitate the resin dispersion in the bath so as to obtain essentially constant concentration of pigmented resin throughout the bath, particularly in the vicinity of the AC electrode from which material is being continuously removed.

The process of the present invention is applicable to coating with all the resins mentioned and described in An acrylic copolymer was prepared having the following composition in proportions by weight:

Acrylamide Methacrylic acid 10 Butyl acrylate 45 Styrene 35 Dodecylmercaptan 1 Ditertiary butyl peroxide 1.5

The four monomers were copolymerized in butanol solution as described in Example 6 of co-pending application Ser. No. 261,564 of 1963 by J. K. Gentles et al. The resulting product was a clear viscous copolymer solution having a solids content of 50% by weight. An emulsion of this copolymer was prepared as follows:

Gm. Acrylic copolymer solution (50% solids) 100 Triethanolamine 9 Water 400 The acrylic resin, the triethanolamine and 200 gms. of water were placed in a beaker fitted with a high speed stirrer. The mixture Was stirred under ambient conditions until the acrylic resin was completely dispersed. This operation took about one minute. The remainder of the water was added with continuous stirring. The total period of agitation was five minutes.

An electrophoretic cell was set up consisting of two fiat steel plates each having a total surface area of sq. inches. The cell was filled with the emulsion described above so that approximately 13 sq. inches of each plate were immersed. A potential difference of 100 volts AC was applied across the cell and an alternating current passed having an initial value of 1.2 amperes which rapidly fell to 0.1 ampere in one minute. On removing the plates they were found to be covered with an adherent layer of acrylic resin, this layer covering all parts of the immersed metal surface including the sharp edges. On heating for 30 minutes at 140 C. smooth even thermoplastic coatings of uniform thickness were obtained including the sharp edges. This example was repeated at higher potential differences as specified in Table 1 and the results specified in Table 1 were obtained.

A pigmented acrylic resin was prepared having the following composition by weight:

Gm. Acrylic copolymer solution in butanol) 59.6 Urea formaldehyde resin solution (57% solids) 10.4 Titanium dioxide pigment 12.0 2 ethoxy ethyl acetate 18.0 Triethanolamine 5.0

Water 233.0

The, acrylic resin, pigment and 2 ethoxy ethyl acetate were ground together to form a mill base. The urea formaldehyde resin was added to this mill base and the paint so formed was placed together with gm. of water and the triethanolamine to a beaker equipped with a high speed stirrer. The mixture was stirred until the paint had completely dispersed. This took approximately one minute. The remainder of the water was added stirring continuously. The total period of stirring was five minutes.

An electrophoretic cell was set up consisting oft wo flat plates as previously described in Example 1. The cell was filled with the emulsion described above. A potential difference of 100 volts AC was" applied across the cell and an alternating current passed having an initial value of 1.2 amperes, which fell rapidly to 0.1 ampere in one minute. On removing both plates they were found to be covered with an adherent layer of pigmented acrylic resin. These films when stoved for 30 minutes at C. produced pigmented non-thermoplastic films having good hardness combined with excellent adhesion to the metal substrate. The example was repeated at potential differences of volts, 200 volts and 250 volts AC gave films of increased thickness averaging one to four thousandths of an inch depending on the potential difference.

Example 7 An alkyd resin was prepared by charging 2.0 moles of trimellitic anhydride, 4.4 moles of propylene glycol and 1.88 moles of dehydrated castor oil fatty acids into a three necked round bottomed flask fitted with stirrer, thermometer and heating apparatus. The mixture was heated to a final temperature of 200 C., using a nitrogen sparge to prevent oxidation, until a viscosity of 6-10 poises (75% solids in 2 ethoxy ethyl acetate) and an acid value of 30-40 (mg. KOH per gram solid resin) was obtained.

An aqueous emulsion was then made up from the following components:

Component: Parts by weight, gm. Titanium dioxide pigment 1 100 Alkyd resin (75 solids) 1 8O 2-ethoxy ethyl acetate 20 Alkyd resin (75% Solids) 133 Urea formaldehyde resin (57% solids) 67.2 Triethanolamine 16.0 Water 1650.0

1 Part A.

A mill base was prepared by grinding the pigment in the alkyd resin and 2 ethoxy ethyl acetate specified as part A. The remainder of the alkyd resin and urea formaldehyde resin were added to this mill base and stirred thoroughly with a mechanical stirrer to form the paint base. The triethanolarnine and 800 gm. of water were added to this paint base and the mixture stirred rapidly to disperse the paint in the triethanolamine and water. This process took approximately one minute. The remainder of the water was added, stirring continuously, and the total period of stirring was five minutes.

An electrophoretic cell was set up consisting of two fiat steel plates each having a toal surface area of 15 sq. inches. The cell was filled with the emulsion described above so that approximately 13 sq. inches of each plate were immersed. A potential difference of 150 volts AC was applied across the cell and an alternating current passed having an initial value of 1.2 amperes which rapidly fell to 0.1 ampere in one minute. On removing the plates they were found to be coated with an adherent layer of pigmented alkyd resin, this layer covering all parts of the immersed metal surface including the sharp edges. On heating for 30 minutes at 140 C. smooth even nonthermoplastic coating of uniform thickness were obtained including the sharp edges. The example was repeated at higher potential differences as specified in Table 2 and the results as specified in Table 2 were obtained.

obtained Without the use of the wire pole. This effectively alters the resistance of the system damping down undesir- Examples 12 and 13 An electrophoretic cell was set up having one fiat steel plate of 15 sq. inches total area as one pole and the second pole being a steel wire. The cell was filled with the pigmented alkyd resin emulsion described in Example 7. A potential diiference of 250 volts AC was applied across the wire and flat plate poles, and an alternating current passed having an initial value of 1.2 amperes which fell rapidly to 0.1 ampere in thirty seconds. This experiment was repeated under the same conditions as Example 7 but continuing the deposition time for one minute.

On removing the flat plate and wire they were found to be coated with an adherent layer of pigmented alkyd resin covering all parts of the immersed metal surface including the sharp edges. The flat plate was heated for 30 minutes at 140 C. and the results obtained were as specified in Table 3.

The wire was easily cleaned for further use by washing in a solvent bath containing xylol/butanol in the ratio of four parts xylol to one part butanol by weight. The thickness of the coating of the wire pole and the flat plate pole were similar and electropheretic deposition by the alternating current process was proportional to the surface area available for coating able electrolysis effects. The voltage drop between any one article and the wire pole is 0 volts.

In a further experiment this was extended to multiple line coatings as shown in FIG. 2. In each case good coatings were obtained even though the voltage diiference between adjacent articles and wire poles was only 125 volts A.C. This was due to the fact that the second Wire pole would be at 375 volts to each of the extreme articles and that these articles are at 250 volts potential to the article on the centre tapping. With the use of intermediate wire poles good coatings were obtained on all articles in -60 seconds.

Example 17 The process is not limited to single phase alternating current but may easily be adapted for three-phase working as shown in FIGS. 3 and 4.

Using a star connection (FIG. 3) with an article connected to each phase and a wire electrode to the null or neutral point three articles may be coated simultaneously. The voltage between each article and the wire pole is 240-250 volts A.C.

In the case of a delta connection (FIG. 4) it is found desirable to use one phase as the wire pole and the other two phases for articles to be coated as the potential differ- TABLE 3 Current Condition of Film after Thickness Time of Imamperes stoving at 140 C. for 1111 Example Voltage merslon for 30 mms. (thousands Initial Final of an inch) 12 250 30 1. 2 0.1 Hard and glossy 0. 8 13 250 60 1. 2 0. 07 Hard and glossy 1. 5

Example 14 ence is 415 volts in each case. Good coatings were ob- This example illustrates the use of voltages 250-500. By means of a step-up transformer, single phase A.C. deposition has been examined up to 500 volts A.C. Good coatings of the order of 0.5-1 thou. inch thickness were obtained. The desirable time of immersion during which current passed was 60 seconds at 250-300 volts and 20-30 seconds at 350-500 volts.

Example 15 This example illustrates the use of various thickness wires as the second pole. Various gauge wires were examined as the second pole in the circuit, the article being connected to the first or live pole, although the reverse connections would apply since the system uses alternating power supply. Good coatings were obtained over a period of 30-60 seconds over the voltage range 250-500 volts, with wires ranging from 8-32 S.W.G. The thinner gauge wires were preferred giving excellent films (28-32 S.W.G.)

Example 16 This example illustrates double line or multiple line production. Further experiments were conducted by connecting articles, one to each end of the secondary windings of a step-up transformer and a third pole being a Wire to a centre tapping on the secondary windings as shown in FIG. 1. The wire pole is eifectively at null voltage. Good coatings were obtained superior to those that would be tained in 30-60 seconds.

The application to 3-phase working allows use of electric power direct without rectification in the case of direct current deposition or the need for step-up transformers in the case of single-phase A.C. deposition.

Example 18 This example illustrates the use of a continuous wire pole. The one disadvantage of the system for coating by alternating current is that all poles are coated including the Wire pole. There is a slight reduction in efliciency of paint usage over direct current deposition but this is outweighed by the elimination of expensive rectification equipment where power consumption is high.

An arrangement has been devised for continuously cleaning the wire pole so that this process becomes a suitable method for use on the industrial scale. The arrangement is shown in FIG. 5.

The continuous wire pole moves through the deposition tank containing the emulsion over a pulley system. It then passes into a solvent tank which contains a mixture'of xylol and butanol in ratio 4:1 by volume. The freshly deposited paint dissolves readily in this tank which is agitated leaving a clean wire for further deposition. All pulleys outside the tank are of normal metal construction and insulated from supporting framework. The pulley immersed in the coating tank is of nylon or similar composition.

1. A process for the deposition of a film of a pigmented resin on a metal object, which comprises dispersing a resin having free acid groups and a pigment in an aqueous medium containing a base to form an aqueous dispersion of pigmented resin, forming an electrolytic cell using the metal object as one electrode of said cell and subjecting the aqueous dispersion to electrophoresis to deposit the pigmented resin on the metal object as an adherent film, the voltage applied to the cell being a symmetrical alternating current voltage.

2. A process for the deposition of a film of a pigmented resin on a metal object,which comprises incorporating a pigment into a resin having free acid groups, dispersing the pigmented resin inan aqueous medium containing a base to form an aqueo'us dispersion of said pigmented resin, forming an electrolytic cell using the metal object as one electrode of said cell and subjecting the aqueous dispersion to electrophoresis by symmetrical alternating current to deposit the pigmented resin on the metal object as an adherent film, the voltage applied to the cell being from 100 to 500 volts.

3. A process as claimed in claim 2 in which the voltage is at least 200 volts.

4. A process as claimed in claim 2 in which the currentdensity is from 8 to 15 amps per square foot.

5. A process as claimed in claim 2 in which the current is passed through the electrophoretic cell for a period of time from 30 seconds to 2 minutes.

6. A process for the deposition of a film of a pigmented resin on a metal object, which comprises dispersing a resin having free carboxylic acid groups and a pigment in an aqueous medium containing a base to form an aqueous dispersion of pigmented resin, forming an electrolytic cell using the metal object as one electrode of said cell and subjecting the aqueous dispersion to electrophoresis to deposit the pigmented resin on the metal object as an adherent film, the voltage applied to the cell being a symmetrical alternating current voltage.

7. A process as claimed in claim 6 in which the resin is an alkyd resin.

8. A process 'as claimed in claim 6 in which the resin is an acrylic resin.

9. A process for the deposition of a film of a pigmented resin on a metal object, which comprises incorporating a pigment into a resin having free acid groups, dispersing the pigmented resin in an aqueous medium containing a base to form an aqueous dispersion of said pigmented resin, forming an electrolytic cell using the metal object as one electrode of said cell, subjecting the aqueous dispersion to electrophoresis by symmetrical alternating current to deposit the pigmented resin on the metal object as an adherent film, and during said electrophoresis rotating the electrodes of the cell relative to one another to reduce surface differences in the deposit.

10. A process for the deposition of a film of a pigmented resin on a metal object, which comprises incorporating a pigment into a resin having free acid groups, dispersing the pigmented resin in an aqueous medium containing a base to form an aqueous dispersion of said pigmented resin, forming an electrolytic cell using the metal object as one electrode of said cell and subjecting the aqueous dispersion to electrophoresis by symmetrical alternating current to deposit the pigmented resin on the metal object as an adherent film, another electrode of the cell being formed from a piece of wire whereby a continuous smooth coating is formed over substantially all the surface of the metal object.

11. A process for the deposition of a film of a pigmented resin on a metal object, which comprises incorporating a pigment into a resin having free acid groups, dispersing the pigmented resin in an aqueous medium containing a base to form an aqueous dispersion of said pigmented re- 8 sin, forming an electrolytic cell using the metal object as one electrode of said cell and subjecting the aqueous dispersion to electrophoresis by symmetrical alternating current to deposit the pigmented resin on the metal object as an adherent film, said aqueous dispersion being continuously agitated during electrophoresis so as to obtain an essentially constant concentration of pigmented resin throughout the cell.

12. A process for the deposition of a film of a pigmented resin on a metal object which comprises incorporating a pigment into an alkyd resin having free ionic acid groups, dispersing the pigmented resin in an aqueous medium containing a base to form an aqueous dispersion of said pigmented resin having a resin solids content of 10% to 20% by Weight, forming an electrolytic cell using the metal object as one of the electrodes of said cell and subjecting the aqueous dispersion to electrophoresis by symmetrical alternating current to deposit the pigmented resin on the metal object as an adherent film.

13. A process for the deposition of a film of a pigmented resin on a metal object which comprises incorporating a pigment into an alkyd resin having free ionic acid groups derived from linseed oil fatty acids, propylene glycol and trimellitic anhydride, dispersing the pigmented resin in an aqueous medium containing a base to form an aqueous dispersion of said pigmented resin, forming an electrolytic cell using the metal object as one electrode of said cell and subjecting the aqueous dispersion to electrophoresis to deposit the pigmented resin on the metal object as an adherent film, the voltage applied to the cell being a symmetrical alternating current voltage.

14. A process as claimed in claim 13 in which the base is ammonia.

15. A process as claimed in claim 13 in which the base is an alkali metal hydroxide.

16. A process as claimed in claim 13 in which the pigment is titanium dioxide.

17. A process for the deposition of a film of a pigmented resin on a metal object, which comprises incorporating a pigment into a resin having free acid groups, dispersing the pigmented resin in an aqueous medium containing a base to form an aqueous dispersion of said pigmented resin, forming an electrolytic cell using the metal object as one electrode of said cell, subjecting the aqueous dispersion to electrophoresis by symmetrical alternating current whereby to deposit the pigmented resin on the metal object as an adherent film, subsequently removing said coated metal object from said electrolytic cell and stoving the film at elevated temperature.

18. A continuous process for coating metal objects with a film of a pigmented resin which comprises continuously subjecting a bath of pigmented resin dispersion to electrophoreses by symmetrical alternating current, said bath comprising a resin having free acid groups and a pigment dispersed in an aqueous medium containing a base, continuously passing an electrode in the form of an endless wire through said bath of pigmented resin dispersion during said electrophoresis, continuously passing the endless wire through a cleaning bath to remove adherent pigmented resin, continuously returning the cleaned wire to the bath of pigmented resin dispersion, and continously passing through the bath during electrophoresis the objects to be coated, said objects forming the other electrode.

19. A process for the deposition of a film of a pigmented resin on a metal object, which comprises dispersing a resin having free carboxylic acid groups and a pigment in an aqueous medium containing triethanolamine to form an aqueous dispersion of pigmented resin, forming an electrolytic cell using the metal object as one electrode of said cell and subjecting the aqueous dispersion to electrophoresis to deposit the pigmented resin on the metal obejct as an adherent film, the voltage applied to the cell being a symmetrical alternating current voltage.

(References on following page) 9 10 References Cited OTHER REFERENCES UNITED STATES PATENTS Cocks, H.C.: Some Possible Uses of Alternating Cur- 2321439 6/1943 Verney et a1 204 181 rents in Electrodeposition, in the Metal Industry, Apr. 19, 2,860,113 11/1958 Botton et al 26022 5 1929 3,200,057 8/1965 Burnside et aL North A.G. Water-D 1lutable Stovmg Flmshes, m 011 & 3,200,058 8/1965 204 181 Colour Chermsts Assoclation Journal, vol. 44, N0. 2, pp. 3,230,162 1/1966 Gilchrist 204-181 Ffibruary 1961- FOREIGN PATENTS JOHN H. MACK, Primary Examiner. 482,548 3/1938 Great Britain. HOWARD WILLIAMS Examine- 514,849 11/ 1938 Great Britain. E. ZAGARELLA, Assistant Examiner. 

